1. Field of the Invention
The present invention relates generally to characterizing metal-oxide-semiconductor structures, and more particularly, to a method for measuring capacitance.
2. Description of Related Art
Capacitance-voltage (CV) measurement is generally used to characterize metal-oxide-semiconductor (MOS) structure of a MOS field-effect-transistor (MOSFET). In particular, a number of critical parameters from the CV measurements are extracted, such as channel length, mobility, interface traps, substrate doping profile, and oxide thickness.
As the manufacturing processes are improved, the device sizes of the MOSFETS are decreasing and the gate insulation or gate oxide is becoming thinner. The thinner insulator of the MOS structure results in a combination of high leakage current and low capacitor impedance. Therefore, the gate capacitance is becoming more difficult to determine.
For example, current capacitance-voltage test systems may be used to obtain CV measurements from a device. These systems use a AC voltage source and a DC biasing voltage source that are added to form a stimulus that is applied through stimulus terminals coupled to substrate region of the device. A sense terminal is coupled to a gate to provide a return path for the currents generated by the stimulus voltages. Using a current meter, the AC current amplitude and phase are measured to determine the capacitance of the test structure. The capacitance at each biasing level is recorded to form a CV curve.
However, the capacitance measurements using the current method and system, especially for thin gate oxides are distorted by the dominance of the large gate leakage current. To minimize this effect, the test structure is made smaller (e.g., less than 100 micrometer2). This causes a further inaccuracy in the measurement of the gate capacitance since the parasitic capacitances now begin to dominate.
The referenced shortcomings are not intended to be exhaustive, but rather are among many that tend to impair the effectiveness of previously known techniques concerning measuring capacitance values; however, those mentioned here are sufficient to demonstrate that the methodologies appearing in the art have not been satisfactory and that a significant need exists for the techniques described and claimed in this disclosure.